Ductless Mini Split

Ductless mini-split systems provide highly efficient heating and cooling without requiring ductwork. These split-system heat pumps consist of an outdoor condensing unit connected to one or more indoor air-handling units via refrigerant lines and control wiring.

System Configurations

Single-Zone Systems

Single-zone systems connect one outdoor unit to one indoor unit, providing independent climate control for a specific space. This configuration offers the simplest installation with minimal refrigerant piping complexity. Capacity ranges typically span 9,000 to 36,000 BTU/hr per zone. Single-zone systems are ideal for room additions, converted garages, sunrooms, or spaces requiring isolated temperature control independent of central HVAC systems.

Multi-Zone Systems

Multi-zone configurations connect one outdoor unit to multiple indoor units through refrigerant branch boxes. The outdoor unit capacity must equal or exceed the sum of all connected indoor units under simultaneous operation. Most manufacturers limit systems to 5-8 indoor units per outdoor unit. Branch boxes distribute refrigerant flow to individual zones with internal metering devices. Multi-zone systems enable whole-house heating and cooling without ductwork while providing independent temperature control in each zone.

Indoor Unit Types

Wall-Mounted Units

Wall-mounted units represent the most common indoor configuration. These units install high on interior walls, typically 7-8 feet above the floor, delivering conditioned air via horizontal louvers and vertical vanes. Installation requires only a 3-inch penetration through the wall for refrigerant lines and condensate drain. Wall units offer the simplest installation with the lowest equipment cost. Airflow rates range from 200 to 800 CFM depending on capacity.

Ceiling Cassette Units

Ceiling cassette units mount flush with suspended or drop ceilings, providing four-way air distribution through a square grille. This configuration offers superior air distribution compared to wall units, with more uniform temperature control throughout the space. Cassettes require adequate plenum space above the ceiling for the unit body and condensate drain pan. These units suit commercial applications, open-plan offices, retail spaces, and residential areas with suspended ceilings. Cassette units maintain a low profile with minimal visual impact.

Floor-Mounted Units

Floor-mounted units install at floor level, similar to traditional radiators or baseboard units. These units discharge air horizontally across the floor or upward along the wall. Floor units are ideal for spaces with limited wall area, rooms with full-height windows, or applications requiring accessibility for individuals with mobility limitations. Installation requires less than 6 inches of floor clearance beneath windows or along walls.

Concealed Duct Units

Concealed duct units mount in ceiling plenums or closets, connecting to short duct runs serving multiple diffusers in a single zone. These units provide a fully concealed appearance with only supply grilles and return grilles visible. Static pressure capability ranges from 0.2 to 0.8 inches w.g., limiting duct length to approximately 25-50 equivalent feet. Concealed units bridge the gap between fully ductless systems and conventional ducted systems for applications requiring aesthetic concealment.

Inverter Compressor Technology

Ductless mini-splits utilize variable-speed inverter-driven compressors that modulate capacity continuously from approximately 20% to 120% of nominal rating. This modulation occurs through DC motor speed control driven by variable-frequency drives. Inverter technology eliminates the cycling losses associated with single-speed compressors, maintaining precise temperature control within ±1°F of setpoint.

The capacity modulation follows the instantaneous load, reducing energy consumption during partial load conditions. Compressor speed adjusts based on the difference between actual space temperature and setpoint temperature. This continuous operation at reduced capacity improves dehumidification during cooling mode by maintaining lower evaporator temperatures for longer durations.

Cold Climate Performance

Modern cold climate ductless heat pumps maintain full heating capacity at outdoor temperatures down to 5°F, with reduced capacity operation continuing to -15°F or lower. Enhanced vapor injection technology and compressor oil management systems enable reliable operation at extreme low temperatures.

Cold climate models incorporate:

• Two-stage or variable-speed compressors with vapor injection ports
• Accumulator heaters preventing refrigerant migration during off-cycles
• Aggressive defrost algorithms minimizing capacity degradation
• Enhanced heat exchangers with increased surface area
• Synthetic oil formulations maintaining lubrication at low temperatures

Heating capacity typically degrades 20-30% between 47°F and 5°F outdoor temperature for cold climate models, compared to 50-60% degradation for standard models.

Efficiency Ratings

Seasonal Energy Efficiency Ratio (SEER)

SEER quantifies cooling efficiency over a typical cooling season. Modern ductless systems achieve SEER ratings between 16 and 33, significantly exceeding minimum federal standards of 15 SEER for split systems. The highest efficiency models incorporate multi-stage or variable-speed compressors with optimized refrigerant circuits.

SEER calculation methodology weights performance at multiple operating conditions including 82°F, 87°F, 92°F, 97°F, and 102°F outdoor temperatures, representing typical cooling season distribution.

Heating Seasonal Performance Factor (HSPF)

HSPF measures heating efficiency across the heating season. Current ductless systems range from 9.0 to 14.0 HSPF, with ENERGY STAR certification requiring minimum 9.0 HSPF in northern regions. HSPF calculation includes cyclic degradation, defrost impacts, and performance degradation at low outdoor temperatures.

HSPF2 represents the updated testing standard effective January 2023, providing more realistic seasonal performance estimates including expanded temperature ranges.

Refrigerant Line Requirements

Line Length Limitations

Maximum refrigerant line length between outdoor and indoor units typically ranges from 50 to 230 feet depending on system capacity and manufacturer specifications. Longer line lengths increase pressure drop, reducing system capacity and efficiency. Each manufacturer publishes maximum allowable line lengths and equivalent line length calculations accounting for fittings and elevation changes.

Minimum line length requirements of 10-15 feet prevent oil return issues and ensure adequate refrigerant charge. Lines shorter than minimum length require additional refrigerant circuit modifications or field-installed accumulators.

Elevation Differences

Vertical separation between outdoor and indoor units affects oil return to the compressor. Maximum elevation differences range from 50 to 165 feet depending on system design. When the indoor unit is positioned above the outdoor unit, oil return becomes critical. Installation with indoor units more than 30 feet above outdoor units typically requires oil traps every 20-30 feet of vertical rise in the suction line.

Installations with outdoor units above indoor units are less restrictive, though maximum elevation limits still apply based on system design pressures.

Line Set Sizing

Refrigerant line sets consist of liquid line and suction line, sized according to system capacity and line length. Undersized lines increase pressure drop, reducing capacity and efficiency. Oversized lines reduce refrigerant velocity, potentially causing oil return issues. Standard line sizes for residential systems include:

Multi-zone systems require larger line sizes and branch boxes with individual metering for each zone.

Applications

Ductless mini-splits serve diverse applications where ductwork is impractical or cost-prohibitive:

Residential retrofits in homes without existing ductwork, including older homes with radiator heating, homes with insufficient space for ductwork, or room additions isolated from central systems.

Supplemental conditioning for problem areas inadequately served by central systems, including converted attics, finished basements, bonus rooms, or spaces with excessive solar gain.

Multifamily buildings where individual metering is required, noise transmission must be minimized between units, or central systems are impractical.

Commercial applications including retail spaces requiring flexible zoning, server rooms requiring redundant cooling, offices with variable occupancy patterns, or tenant spaces requiring independent control.

Temporary or modular structures where permanent ductwork installation is not justified, including construction trailers, portable classrooms, or seasonal facilities.

The independent zone control, high efficiency, and installation flexibility make ductless mini-splits suitable for both new construction and retrofit applications across residential and light commercial sectors.